An electric power steering apparatus which provides a steering system of a vehicle with a steering assist torque (an assist torque) by means of a rotational torque of a motor, applies the steering assist torque to a steering shaft or a rack shaft by means of a transmission mechanism such as gears or a belt through a reduction mechanism. In order to accurately generate the assist torque, such a conventional electric power steering apparatus performs a feedback control of a motor current. The feedback control adjusts a voltage supplied to the motor so that a difference between a steering assist command value (a current command value) and a detected motor current value becomes small, and the adjustment of the voltage applied to the motor is generally performed by an adjustment of a duty of a pulse width modulation (PWM) control.
A general configuration of the conventional electric power steering apparatus will be described with reference to FIG. 1. As shown in FIG. 1, a column shaft (a steering shaft or a handle shaft) 2 connected to a handle (steering wheel) 1 is connected to steered wheels 8L and 8R through reduction gears 3, universal joints 4a and 4b, a rack-and-pinion mechanism 5, and tie rods 6a and 6b, further via hub units 7a and 7b. In addition, the torsion bar is interposed within the column shaft 2, the column shaft 2 is provided with a steering angle sensor 14 for detecting a steering angle θ of the handle 1 by means of a torsional angle of the torsion bar and a torque sensor 10 for detecting a steering torque Th of the handle 1, and a motor 20 for assisting a steering force of the handle 1 is connected to the column shaft 2 through the reduction gears 3. The electric power is supplied to a control unit (ECU) 30 for controlling the electric power steering apparatus from a battery 13, and an ignition key signal is inputted into the control unit 30 through an ignition key 11. The control unit 30 calculates a steering assist command value of an assist (a steering assist) command on the basis of a steering torque Th detected by the torque sensor 10 and a vehicle speed Vel detected by a vehicle speed sensor 12, and controls a current supplied to the motor 20 by means of a voltage control command value Vref obtained by performing compensation or the like to the steering assist command value. It is possible to receive the vehicle speed Vel from a controller area network (CAN) or the like.
A steering angle sensor 14 is not indispensable and may not be provided. It is possible to obtain the steering angle from a rotational position sensor which is connected to the motor 20.
The controller area network (CAN) 40 to send/receive various information and signals on the vehicle is connected to the control unit 30, and it is also possible to receive the vehicle speed Vel from the CAN. Further, a Non-CAN 41 is also possible to connect to the control unit 30, and the Non-CAN 41 sends and receives a communication, analogue/digital signals, electric wave or the like except for the CAN 40.
The control unit 30 mainly comprises a CPU (Central Processing Unit) (including an MPU (Micro Processor Unit) and an MCU (Micro Controller Unit)), and general functions performed by programs within the CPU are, for example, shown in FIG. 2.
The control unit 30 will be described with reference to FIG. 2. The steering torque Th detected by the torque sensor 10 and the vehicle speed Vel detected by the vehicle speed sensor 12 (or from the CAN 40) are inputted into a torque control section 31 to calculate a current command value Iref1. The torque control section 31 calculates the current command value Iref1, which is a control target value of a current supplied to the motor 20, based on the steering torque Th and the vehicle speed Vel using an assist map or the like. The current command value Iref1 is inputted into a current limiting section 33 via an adding section 32A, the current command value Irefm that is limited the maximum current, is feed-backing-inputted into a subtracting section 32B. A deviation I (=Irefm−Im) between the current command value Irefm and a motor current value Im is calculated and the deviation I is inputted into a current control section 35, which performs a proportional-integral (PI)-control and so on, for improving a characteristic of the steering operation. The voltage control command value Vref that the characteristic is improved at the current control section 35, is inputted into a PWM-control section 36, and the motor 20 is PWM-driven through an inverter 37. The current value Im of the motor 20 is detected by a motor current detector 38 and is fed-back to the subtracting section 32B. The inverter 37 is constituted by a bridge circuit of field-effect transistors (FETs) serving as semiconductor switching devices.
A rotational sensor 21 such as a resolver is connected to the motor 20 and a motor rotational angle θ is outputted.
A compensation signal CM from a compensation signal generating section 34 is added at the adding section 32A. A characteristic compensation of the steering system is performed by adding the compensation signal CM, and then a convergence, an inertia property and so on are improved. The compensating section 34 adds a self-aligning torque (SAT) 343 with an inertia 342 at an adding section 344, further adds the result of addition performed at the adding section 344 with a convergence 341 at an adding section 345, and then outputs the result of addition performed at the adding section 345 as the compensation signal CM.
In such an electric power steering apparatus, recently, the vehicles, which have the automatic steering control function such as the parking assist function (parking assist), the lane keeping function and so on, and switch between the automatic steering control and the manual steering control, have been appeared. In the vehicles having the parking assist function, the target steering angle is set based on data from a camera (an image), a distance sensor and so on, and the automatic steering control that an actual steering angle follows a target steering angle, is performed.
In the conventional electric power steering apparatus having a well-known automatic steering control function, a back-in parking or a parallel parking is automatically performed by controlling the motor based on a relationship between a pre-stored traveling distance of the vehicle and a turning steering angle.
FIG. 3 shows a configuration example of a control system of the vehicle having the conventional automatic steering control function, and the control system of the vehicle comprises a torque control section 110 to calculate the current command value Itref by inputting at least the steering torque, an automatic steering control section 120 to calculate the current command value Isref by inputting the target steering angle or the like from the ECU of the vehicle side, and a switching section 130 to output as the current command value Iref by switching the current command value Itref or Isref depending on a switching signal SW from the ECU of the vehicle side. The current command value Iref is inputted into a current control/driving section 140, and the current control/driving section 140 PWM-controls the motor 20 with a PWM-signal being performed the PI-control or the like.
In such a vehicle having the automatic steering control function, when the driver operates the handle during the automatic steering control function and it is judged that the steering torque is greater than a predetermined value being stored in advance, the automatic steering control is conventionally stopped.
However, if the judging is performed only by comparing the output of the steering torque detecting means with the predetermined value, the output of the automatic steering detecting means becomes temporarily higher than the predetermined value due to a noise of the steering torque detecting means, or an inertia torque of the handle when the tire steps on a small stone or the automatic steering is performed by means of the motor. Thus, there is a problem that the automatic steering control is stopped at each time. In order to avoid such a disadvantage, when the predetermined value is set higher, not only the driver feels uncomfortable since the automatic steering and the manual steering interfere with each other, but also the automatic steering control cannot be immediately stopped even if the driver operates the handle during the automatic steering control.
Then, when the steering torque being the predetermined value or more is detected over a predetermined time or more, it is thought that the control system judges the performance of the manual steering and then stops the automatic steering control. In this case, when the driver operates a moderate manual steering and the steering torque is slightly higher than the predetermined value, the driver does not feel uncomfortable and the automatic steering control is stopped after the predetermined time is passed. However, in a case that the driver operates the rapid manual steering and then the steering torque is considerably higher than the predetermined value, the handle is not steered smoothly and the driver may feel uncomfortable because the automatic steering is not stopped until the above state elapses the predetermined time.
As an apparatus for resolving the above problem, for example, Japanese Patent No. 3845188 B2 (Patent Document 1) is proposed. The apparatus disclosed in Patent Document 1 comprises a steering torque detecting means to detect a steering torque which a driver applies to a handle, and a motor control means to control the driving of the motor based on a traveling track which is set by a traveling track setting means and to stop the motor control due to the traveling track when the steering torque being a predetermined value or more is detected over a predetermined time or more. Then, the apparatus sets plural kinds on the predetermined value and changes the predetermined time corresponding to the respective predetermined values.